To decorate injection-molded shaped parts with printed polymethyl methacrylate films (PMMA) there are used PMMA films with thicknesses in the range between 80 and 300 xcexcm for the purpose of achieving a certain xe2x80x9cdepth effectxe2x80x9d of the printing. For decoration, either printed PMMA films colaminated with thermoplastic laminate substrates are preformed and stamped to the desired geometry then placed in the injection-molding die and back-molded, or printed PMMA film in the form of rolled stock is guided into an injection-molding die and back-molded.
Back-molding takes place on the printed side, so that the printing is protected by the PMMA film, which contains UV absorbers. Examples of materials for use in back-molding include the following thermoplastics and polymer blends: ABS, PC/ABS blends, PVC, PC, PC/ASA blends, PP, PP blends.
For decoration, either printed film-type formed parts (optionally preformed) are placed in the injection-molding die and back-molded, or printed PMMA film in the form of rolled stock is introduced into the injection-molding die and back-molded, or PMMA film is laminated onto a thermoplastic laminate substrate and (optionally preformed) placed in the injection-molding die and back-molded. As laminate substrates which can be laminated with the inventive and printed film, there can be used, for example, the following thermoplastics: ABS, polycarbonate-ABS blends, ASA (copolymers of acrylic ester, styrene and acrylonitrile), polycarbonate-ASA blends, polyolefins such as polyethylene and polypropylene, PVC and polyphenylene-containing blends. All thermoplastic materials can be reinforced by glass fibers or inorganic fillers.
The PMMA film assumes the function of a protective lacquer. In contrast to injection-molded parts which are protected with protective lacquer or which are directly printed or printed by transfer methods (thermal transfer, water-based transfer), surface decoration with printed PMMA films offers the following advantages:
clear cost reduction (surface finish and decoration are achieved in one process step)
solvent-free process
no expensive capital spending and environmental directives
simple change of the printed design
greater design freedom
The decoration of thermoplastic shaped parts with printed films concurrently with the injection-molding process is generally known as xe2x80x9cin-mold film decorationxe2x80x9d. The best known form of this method is called xe2x80x9cinsert moldingxe2x80x9d. For this purpose the decoratively printed PMMA film is stiffened by colamination with a thermoplastic laminate substrate. For this purpose there are preferably used ABS, ASA, PC, PVC, PP, PPE and blends of these materials.
The film laminate is formed and stamped to the desired geometry by means of a thermoforming process. The preforms are then fed to the back-molding process for actual manufacture of the shaped parts. For this purpose the preforms are placed in the injection-molding die, for the most part automatically, and back-molded with thermoplastic plastics. For back-molding there are preferably used ABS, ASA, PC, PVC, PP, PPE and blends of these materials. If the laminate substrate is used in sufficient thickness, there is no need for back-molding and the unformed, stamped or cut laminate can be used directly as the shaped part.
By virtue of the extremely streamlined process control and the possibility of rapid design change, xe2x80x9cin-mold film decorationxe2x80x9d offers the processers or manufacturers of shaped parts substitute options for more traditional, more complex methods.
For example, PMMA film printed with transparent elastic lacquers then back-molded with transparent thermoplastics can be used in extremely rational manner for the manufacture of automobile tail-light lenses. As the transparent thermoplastics there are preferably used PMMA molding compounds, SAN, polystyrene molding compounds, polycarbonate and PMMA/SAN blends.
This method offers the possibility of rapid design change without having to build new, costly multicomponent injection-molding dies. Moreover, the complex step of coloration of the PMMA molding compounds which are complex for multicomponent injection molding is obviated.
In this type of application, xe2x80x9cin-mold film decorationxe2x80x9d permits the manufacture of complete external parts for automobile bodies, which parts can optionally contain the tail-light lens as a functional surface.
In this application the PMMA film printed with transparent elastic lacquers can optionally be back-molded, prior to the process of manufacture of the shaped part, together with transparent, thermoplastic laminate substrates in order to increase stiffness for protection of the lacquer from the hot, pressurized, molten thermoplastic material used for back-molding.
A special embodiment of xe2x80x9cin-mold film decorationxe2x80x9d which operates very economically uses combined mechanical forming and back-molding dies, with which both mechanical forming of the films and back-molding take place in one process on the basis of printed films in the form of rolled stock. This method is generally known as xe2x80x9cfilm-insert moldingxe2x80x9d.
In the use of PMMA films for decoration of injection-molded shaped parts by means of xe2x80x9cin-mold film decorationxe2x80x9d, exacting requirements are imposed on the PMMA film material:
good ease of manipulation in the printing process, or in other words adequate extensibility
high surface hardness (at least pencil hardness HB), to prevent scratching of the shaped part decorated with PMMA film
high surface gloss of xe2x89xa7120 (measuring technique per DIN 67530)
low haze development to less than 2% haze, corresponding to the method described on page 17
low surface haze of  less than 1.5%
extremely low gel-body count of at most 1 gel body per 200 cm2 
high weathering resistance; comparable with that of standard PMMA
adequate absorption of UV light; UV absorption in the region between 290 and 370 nm  less than 1%.
German Patent DE 3842796 (Rxc3x6hm GmbH) describes PMMA films based on PMMA molding compounds with small-sized elastomer particles and high elastomer content. As regards the xe2x80x9csurface hardnessxe2x80x9d parameter, the inventive films cannot be manufactured with these molding compounds.
International Patent WO 96/30435 and European Patent 763560 (Mitsubishi Rayon) describe the manufacture of PMMA films with thickness up to 0.3 mm on the basis of a specified PMMA composition: an impact-strength modifier based on polybutyl acrylate with specified particle diameter as well as PMMA matrix polymer III and the (optional) addition of a melt-strength modifier (polymer I).
The film is manufactured by means of a single-roll process (otherwise known as the chill-roll melt-casting process), in which the thermoplastic melt is brought into contact with a single metal roll and cooled during the cooling and solidification process. It is expressly pointed out that the thermoplastic melt for manufacture of films of the claimed thickness range cannot be formed between two metal rolls.
Compared with the two-roll process this method suffers from significant disadvantages, which have a decisive influence on film quality. In forming on the single chill roll, in contrast to the two-roll (smoothing) process, gel bodies, which in principle tend to be formed by impact-strength-modified PMMA molding compounds, are not forced underneath the film surface and thus remain visible as optical defects. This is particularly detrimental as regards the subsequent printing process for the manufacture of decorated films, in which clearly visible flaws are apparent in the region of the gel bodies. Furthermore, the film surface opposite the chill roll and cooling naturally in the air exhibits pronounced surface haze, which results from the differences between volume contraction of the elastomer particles and the PMMA matrix. Hereby there is produced a pronounced xe2x80x9cpeak-and-valleyxe2x80x9d surface structure, which scatters light and thus causes a detrimental haze effect.
German Patent DE 19544563 (Rxc3x6hm GmbH) describes the impact-resistant PMMA molding compounds used to manufacture the inventive films.
German Patent DE 4018530 (Rxc3x6hm GmbH) describes a method for manufacture of solid plates or films with thickness less than 1 mm from a thermoplastic plastic with a glass transition temperature above 50xc2x0 C. Smoothing is achieved by guiding the film on an endless belt. The obtained plates or films are practically free of orientations and double refraction.
European Patent EP 659829 (Rxc3x6hm GmbH) describes a weathering-protection film and thus coated shaped parts, wherein the film has the function not only of protecting against weathering but also absorbing UV radiation. It comprises a hard phase of PMMA and a tough phase, the UV absorber being contained in the hard phase.
European Patent 391193 (Bayer AG) describes a method for manufacture of optically isotropic extruded films which are glossy on both sides and have a thickness of less than 0.6 mm, and which either
1. are manufactured by extrusion and subsequent calendering between a lacquered elastic roll and a high-gloss steel roll or
2. are formed in two extrusion steps, the first step being production of a high-gloss film on the one side and a matt film on the other side by extrusion and subsequent calendering between a ground elastic roll and a high-gloss steel roll. In a second extrusion step the film produced in the first step is coated with the melt of the same thermoplastic plastic on the matt side of the film, and the resulting coated film is calendered once again between a high-gloss steel roll and a ground elastic roll, the high-gloss side of the coated film being directed toward the roll of ground elastic material.
The two methods, which not only are technologically complex but also incur extremely high production costs, take into consideration the current prior art, which considers the manufacture between two high-gloss steel rolls of films which are glossy on both sides to be unrealistic because of the extremely large forces, which are difficult to control, in the roll gap.
Method 1 suffers from the disadvantage that it cannot be achieved on a large industrial scale, since the lacquer layers on the rubber rolls become brittle very rapidly under the influence of the high melting temperature. To reduce the influence of the high melting temperatures, the lacquered rubber rolls can be cooled in a water bath, but the moisture leads to a detrimental influence on the surface quality of the film.
Method 2 is extremely unfavorable from the economic viewpoint, since the film must be manufactured in two extrusion steps. Furthermore, the extrusion coating of a film with melt and the subsequent calendering, especially in the thickness range claimed according to the invention, leads to unfavorable surface characteristics.
European Patent 195075 (Exxon) describes a method for manufacture of a film comprising 10 to 85 wt % of an elastomer and 90 to 15 wt % of a polyolefin, wherein the extruded web is passed through the gap between contra-rotating rolls at a temperature above its softening point. One of the rolls is a high-gloss chill roll and the other roll is a roll with a high-gloss rubber surface, thus cooling the film.
The films obtained in this way have a thickness of between 25 and 250 microns (10xe2x88x926 m). Locking forces are not described, and the disadvantages cited in the discussion of European Patent EP 391193 exist here also.
European Patent 212355 (Bayer AG) describes a polycarbonate film which is printed if necessary with an adhesive-free polyurethane layer. The films are matted on one side or structured on one side by drawing them over a matted or structured chill roll. The films obtained in this way are printed and back-molded. A copolymer of acrylonitrile, butadiene and styrene is used as the plastic for back-molding.
European Patent EP 294705 (Rxc3x6hm GmbH) describes a method for the manufacture of films smoothed on both sides, which method uses as a smoothing element an already smoothed film produced previously in the method and recycled.
A. Huemer (Kunststoffe, 87 (1997), 10, pp. 1351 ff.) discusses the advantages and disadvantages of glazing rolls, as does H. Gross in Kunststoffe 87 (1997), 5, p. 564.
Huemer states that xe2x80x9cExperiments with relatively high pressure per unit length fail, because thereby neither the dwell time in the roll gap nor the relaxation time in the polymer changes.xe2x80x9d
The measures described by Huemer (correct ratio of nozzle outlet velocity and drawdown velocity) are not sufficient to obtain films with high surface quality. Contrary to Huemer""s observation, it is not the lowest possible but instead the highest possible pressure per unit length in the roll gap that is necessary to smooth the surface.
The need therefore existed to provide an economically operating method for manufacturing films with thicknesses of xe2x89xa60.3 mm from impact-resistant PMMA molding compounds, which method can be manipulated on a large industrial scale and which also ensures a surface quality that is highly glossy on both sides, is almost free of gel bodies and has surface hardness adequate for decorative applications. Furthermore, the film must be reliably and economically manipulable in the printing process as well as during the xe2x80x9cin-mold decorationxe2x80x9d process, and accordingly it must have excellent extensibility.
Thus extreme value had to be placed on proper selection of the PMMA components, especially as regards the balance between surface hardness and extensibility.